Generally, a semiconductor memory device is classified into a volatile memory device and a non-volatile memory device. The most volatile memory device is a dynamic random access memory (DRAM) and a static random access memory (SRAM), which is characterized in that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was therein is lost. Meanwhile, the non-volatile memory device almost composed of a read only memory (ROM) can hold its contents whenever power is not supplied.
At present, the non-volatile memory device is classified into a floating gate type and a metal insulator semiconductor type, accumulated with 2 dielectric layers or 3 dielectric layers, in aspect of a manufacturing process.
The floating gate type memory device implements a data holding using a potential well, and is represented by an erasable programmable read only memory (EPROM) tunnel oxide (ETOX) structure which is widely used in electrically EPROM (EEPROM).
In the meanwhile, MIS type stores data using a trap existed in a dielectric bulk, a dielectric-dielectric interface, and a dielectric-semiconductor interface. A metal/silicon oxide-nitride-oxide structure is a representative structure used in a flash EEPROM.
As a technology gets advanced, a system on chip (SOC) technique gets highlighted. Herein, the SOC technique means that various functions, such as Logics, Sonos, and Flasys are operable in single chip. However, these functions require a complex processing for implementing thereof.